The present invention relates to a new class of environmental sensors based on the properties of field structured composites. Such composites possess a substructure of ordered aggregates of suspended conducting magnetic particles. This substructure self-assembles under the influence of an external magnetic field, and induces a wide range of mechanical, dielectric, magnetic, and optical properties which can be harnessed to make sensors. These new materials enable a broad range of sensor devices and other applications.
The conduction of electricity in materials comprising a particulate conducting phase dispersed in a nonconducting medium have been of scientific and practical interest for some time. Such materials as conductive inks, some forms of conducting polymers, and static elimination materials have long used such dispersions to provide conductivity to conventionally non-conducting elements.
Such composites have been formed of a composite mixture of conducting particles essentially uniformly distributed in a nonconducting medium. Roughly speaking, one expects the conductivity of a composite mixture to increase as the volume fraction of the dispersed conducting phase increases (i.e., as more conductive particles are introduced into the mixture). This is true, but the bulk conductivity of the composite is not simply proportional to the volume fraction of the dispersed phase. In fact, there is a rather well defined point at which long-range conducting paths appear, called the critical volume fraction.
Near the critical volume fraction, there are many conducting paths that are only interrupted by a few instances where current conduction must go through particles which are nearly, but not quite in contact. Small changes in the particle volume fraction can complete many of the paths, making the conductivity of these materials very sensitive to such changes.
A typical prior art sensor based on such essentially uniform composite materials appears in U.S. Pat. No. 5,574,377. Here a chemical sensor is implemented by measuring the electrical resistance of a composite material formed of a gel-like polymer containing dispersed conducting particles with volume fraction near the critical volume fraction. The sensor material has large conductivity in the absence of external chemicals. However, the sensor material (more particularly the nonconducting polymer) swells when in the presence of certain organic solvents. Such swelling increases the gaps between particles, thereby driving a large reduction in the bulk conductivity of the sensor material. Such chemical sensors can be quite sensitive if the proper volume fraction is achieved in the sensor material.
Despite the clear potential for using such conducting composite materials for a variety of sensor functions, practical applications have been limited by prior art fabrication technology. It is very difficult to disperse conducting particles uniformly in a nonconducting medium. Exceedingly small changes in process conditions, or simply random variations in the local volume fraction of the conducting particles, can reduce or destroy the desired material response.
Thus, conducting composite materials made using conventional technology cannot be routinely applied to most sensor applications unless a great deal of effort is taken to control and then characterize the composite. Numerous samples must be typically made under slightly varying conditions, and the samples then individually characterized in a search for individual pieces having the proper bulk properties. When such composites can be used, the device or mechanism thereby enabled usually requires individual calibration.
There is thus a longstanding need for sensor devices which exhibit the special properties of composite materials comprising conducting particles, but which can be reliably manufactured to exhibit precise and predefined device properties.
The present invention addresses the aforementioned need by substituting for prior art conducting composite materials a new class of conducting composite materials, called field-structured composites (FSC) in which the suspended conducting magnetic particles form an ordered aggregate structure within the nonconducting medium. This structure can be controlled during fabrication to yield a precise and predetermined composite structure, avoiding the prior art fabrication difficulties.
Such a field-structured composite can be made beginning with a nominally uniform dispersion of magnetic particles in a nonconducting fluid that can be solidified. This initial dispersion is typically chosen to have a volume fraction well below the critical volume fraction for a uniform distribution. To form a uniaxial field-structured composite, an external magnetic field of fixed orientation is applied to this initial dispersion. This magnetic field aligns the particles into thin rod-like aggregations of particles which form an interconnecting network within the fluid (see FIG. 1a). This network of particles will conduct, even though the sample is beneath the critical volume fraction for a uniformly dispersed material. Conduction perpendicular to the rods is enabled by the presence of conducting bridges between rods.
A distinct type of FSC can be fabricated by rotating the initial dispersion within the magnetic field around an axis perpendicular to the magnetic field direction. In this case, the particles aggregate into thin sheets of closely packed particles, with relatively few interconnections between these sheets. Both the rod-like aggregations and the sheet-like aggregations are examples of ordered aggregate structures.
The ordered aggregate structure of an FSC can be precisely controlled during fabrication by controlling the process parameters. These process parameters include the time the dispersion is subjected to the structuring magnetic field, the strength of the magnetic field, and the direction of the magnetic field as a function of time. When the desired ordered aggregate structure has been attained, the fluid can be solidified (e.g., by freezing or polymerizing).
Beyond such issues of fabrication control, however, the basic nature of the transition between the nonconducting and the conducting states in a field-structured composite is fundamentally distinct from that in a uniform dispersion of conducting particles. This is a result of the highly anisotropic local structure in field-structured composites. Control over this structure can allow fabrication of composites in which the average bulk conducting path is interrupted by fewer weak conducting links between particles than is found in uniform dispersions. Such structures are dramatically more sensitive than are uniform dispersions in the type of sensor applications included in the present invention.